It is known to mix recycled asphalt products (“RAP”) with virgin aggregate to produce hot mix asphalt (“HMA”). The virgin aggregate is typically first heated and dried in a rotary drum of a counterflow dryer and then mixed with RAP and liquid asphalt in a separate mixing chamber of the counterflow dryer to form a hot asphalt mix suitable for paving. One such counterflow dryer, manufactured by Astec Industries, Inc. and commonly known as a “double barrel dryer”, is schematically illustrated in FIG. 1. The counterflow dryer 10 illustrated in FIG. 1 comprises an inner drum 12 and a fixed outer sleeve 24 mounted on a common frame 16 in an inclined manner. The inner drum 12 is rotatably mounted on the frame 16 by a plurality of bearings 18 and is driven to rotate by a suitable motor 20. A burner 22 directs a flame 24 generally axially into the interior of inner drum 12.
Inner drum 12 has at its first (upper) end 26 a virgin aggregate inlet 28 and a combustion products outlet 30 and has at its second (lower) end 32 a plurality of openings 34 forming heated and dried virgin aggregate outlets. Inner drum 12 also supports a plurality of paddles 36 extending into a mixing chamber 38 formed between the inner drum 12 and the outer sleeve 14. The interior of the inner drum 12 is functionally separated into a combustion zone located in the vicinity of the burner flame 24 and a drying zone located between the combustion zone and the first end 26 of the drum 12.
Outer sleeve 14 is separated from the inner drum 12 by a sufficient distance to form a mixing chamber 38 which is sufficiently wide to provide clearance for the paddles 36. Outer sleeve 14 has an upper RAP inlet 40, a virgin aggregate inlet 42 cooperating with the openings 34 of the inner drum 12, and an asphalt mix outlet 44. Outer sleeve 14 also receives suitable equipment (not shown) for injecting liquid asphalt into the mixing chamber 38.
In use, virgin aggregate is fed into the virgin aggregate inlet 28 of the inner drum 12 via a suitable conveyor 46 and is heated and dried as it travels downwardly through the inclined drum 12 counter to the direction of the flame 24 from the burner 22. Heated and dried aggregate in the second end 32 of the drum 12 falls through openings 34 in the drum 12, through the inlet 42 in the sleeve 14, and into the mixing chamber 38. RAP is simultaneously fed into mixing chamber 38 from the sleeve inlet 40 by a suitable conveyor 48 and is mixed by the paddles 36 with the heated and dried virgin aggregate. Liquid asphalt is also normally injected at this time, thereby forming an asphalt paving mix. In addition to mixing the virgin aggregate, RAP, and liquid asphalt, the paddles 36 also convey the resulting mix to the mixing chamber outlet 44, where the mix is discharged from counterflow dryer 10. Combustion products formed during operation of counterflow dryer 10 rise out of the inner drum 12 through outlet 30 and are conveyed to a downstream device such as a baghouse.
It is also know to use a counterflow dryer in combination with a parallel flow dryer to produce HMA. Conventional HMA plants employing a counterflow and a parallel flow dryer use the parallel flow dryer to heat and dry the RAP, and the counterflow dryer to heat and dry the virgin aggregate and to mix the virgin aggregate with liquid asphalt cement.
However, conventional hot mix asphalt plants utilizing counterflow dryers and/or parallel flow dryers to produce HMA suffer from several disadvantages. For example, conventional plants are limited in the percentage of RAP that may be used to produce HMA. Conventional plants also generate and emit smoke and other harmful emissions produced by RAP. Conventional plants are expensive to operate and produce HMA having inferior quality. Further, conventional plants require significant heating and drying loads to be placed on the parallel flow dryer. Still further, conventional plants do not recycle a large proportion of the gases produced by the parallel flow dryer. In addition, conventional plants do not separate RAP according to gradation. Conventional plants also do not control the characteristics of the HMA produced by the plant by controlling the gradation of RAP used. Further, conventional plants do not introduce RAP into different dryers depending upon the gradation of the RAP. Still further, conventional plants do not sufficiently reduce airborne RAP particles or sticking between RAP particles and the equipment of the plant.
It would be desirable, therefore, if an apparatus and method for a hot mix asphalt plant could be provided that would use a high percentage of RAP. It would also be desirable if such a plant could be provided that would reduce smoke and other harmful emissions produced by RAP. It would be further desirable if such a plant could be provided that would produce high quality, cost-effective hot mix asphalt. It would be still further desirable if such a plant could be provided that would reduce the heating and drying loads placed on the parallel flow dryer. In addition, it would be desirable if such a plant could be provided that would recycle a large proportion of the gases produced by the parallel flow dryer. It would be also be desirable if such a plant could be provided that would separate RAP according to gradation. It would be further desirable if such a plant could be provided that would control the characteristics of the HMA produced by the plant by controlling the gradation of RAP used. It would be still further desirable if such a plant could be provided that would introduce RAP into different dryers depending upon the gradation of the RAP. Additionally, it would be desirable if such a plant could be provided that would reduce airborne RAP particles. It would also be desirable if such a plant could be provided that would reduce sticking between RAP particles and the equipment of the plant.